The invention relates to a method and/or device to detect with the aid of a light beam the relative or absolute position on the displacement axis of a graduation support in relation to a support belonging to a device, which includes:
a light source producing a light flux, PA1 a graduation support having a graduation placed in the light flux which it modifies, PA1 a support containing one or several sensitive elements capable of intercepting the modified light flux, PA1 one or several sensitive elements which convert the intercepted light intensity into a physical effect, PA1 and a detection circuit suited to produce a useful electric circuit for the design of movement and position sensors.
EP Patent application 474 149 A2 (Kawamura) describes such a device in the form of an incremental encoder. The working principle is characterised by the fact that the graduation is made out of a transparent half-division and a focusing half-division to which two sensitive diodes correspond to the light of a width equivalent to a half-division. The level of resolution reached by the device is not quantified.
The dimension of the light spot is defined as infinitely small and the non-illuminated zone as dark. This point of view holds as long as the optics are perfect and the effect of diffraction are not apparent.
The working of the device depends on, among other things, the dimension of the light spot projected by the focusing part on the detection diodes. For example, for a lens of 20 .mu.m and a focal distance of 80 .mu.m which corresponds to the dimension of 40 .mu.m of a division, the width of the light spot corresponds to 30% of the width of the sensitive element. The current curves described in the Kawamura's patent are no longer usable, in particular the resolution can not be multiplied as specified.
The unit resolution of the device corresponds to an alternation of the digital signal for a division length. The sensor's resolution is defined as the graduation's resolution.
The unit resolution corresponds to the resolution defined by the geometric and physical layout of the sensitive elements. The graduation resolution corresponds to the resolution defined by the dimension of a graduation's division.
Another disadvantage of this device lies in the fact that the form of the position signal is not in tune with current technology which is an handicap to the product's commercialization.
Furthermore, the Kawamura disclosure doesn't report the constraints relative to the creation of the light source. It also doesn't mention preferential forms of the lens for improving the device's resolution.
Patent EP 0206 656 (Leonard) describes a device which works with a graduation made of elements having a first half-portion to let the light pass through and a second half-portion to retain the light. A miniaturisation of the graduation support is limited, either for mechanical construction considerations or for the high costs related to the making of a glass graduation using lithographic techniques.
Patent EP 0489399 A2 (Igaki) uses a series of optical elements arranged on a cylinder and a light flux crossing the same graduation twice. On the first crossing, half of the light flux is reflected and the other half is modified by grating effect and projected on the opposite side of the graduation where the light flux is divided in three directions. The possible resolution depends on the wavelength and the graduation-support diameter defined by the formula: EQU (N-1/4)*(graduation pitch)*(graduation pitch)/(wavelength)&lt;graduation diameter&lt;(N+1/4)*(graduation pitch)*(graduation pitch)/(wavelength)
This means that 15 mm diameter corresponds to a resolution of approximately 760 divisions. The commercial product claims a resolution of approximately 80'000 divisions which requires a complex circuit detector for the treatment of the signals generated by three receiver diodes. The design of the graduation support as a cylindrical graduation only doesn't allow, for example, to design a level sensor.
The U.S. Pat. No. 4,531,300 shows an example of a level-measurement device which uses an identical technique to that of Leonard. This device is particularly cumbersome and too costly to compete with the standard bubble-sensor products. The use of capacitors or magnetic-resistant sensors requires an analog/digital circuit converter of higher than 12-bit resolution which is more difficult to conceive than a purely-digital circuit.
Patent PCT/EP 93/02415 describes a kilometer measurement system requiring a sensor for the measurement of the movement dynamics of a vehicle. The use of a gravitational sensor is favorable for its stable functioning over time and its simplicity of use. The values to be measured require a high-resolution sensor which is not available on the market.
With the devices described above it is not possible to conceive small-sized absolute sensors based on a linear-code reading as described in the patent JP 3-6423 (15). There is also not possible to conceive high-resolution gravitational sensors that certain market applications require.